The present invention relates generally to direction sensors for route guidance systems for vehicle navigation and more specifically, to calibration of a relative heading sensor such as a gyroscope.
Frequently, route guidance systems navigate using dead-reckoning, wherein a vehicle's current position is determined by considering a known previous position of the vehicle and the direction and distance travelled since the previous position. In such dead-reckoning systems, it is necessary to track the heading or direction of the vehicle relative to a reference direction. Usually, such systems utilize both an absolute heading sensor, such as a magnetic compass or geomagnetic sensor, for sensing vehicle heading relative to a reference direction fixed with respect to the earth, as well as a relative heading sensor, such as a gyroscope, for sensing vehicle heading relative to a previous vehicle heading, i.e., the change in vehicle heading.
In automobile route guidance systems, the accuracy of such heading sensors can be significantly influenced by a number of factors. For example, the presence of local magnetic fields can significantly affect magnetic compass readings. Such local magnetic fields are created by, for example, mechanisms in the vehicle in which the compass is carried, other vehicles in the vicinity, or nearby structures. Without periodic calibration to compensate for local effects, a magnetic compass will produce heading readings with significant error.
A geomagnetic sensor typically has two outputs from two separate windings which produce X and Y component signals corresponding to the Earth's magnetic field in two directions at right angles from each other. Ideally, if one was to plot the sensor's X and Y outputs at every point as the geomagnetic sensor is turned in a complete circle, the output would be a perfect circle such as shown in FIG. 4. Thus, the direction a vehicle is pointing can be determined by comparing the X and Y coordinates output from the sensor to the center of the circle. In practice, however, the circle typically becomes an ellipse such as shown in FIGS. 5 or 6, due to effects discussed above.
Relative heading sensors, such as gyroscopes, also require periodic calibration. Typically, two constants are critical in determining a relative heading from such a sensor. One is the sensor output at a reference or zero position, commonly called the "zero reading". The zero reading is subject to change with operating temperatures and other environmental factors. An inaccurate zero reading will cause all relative heading readings to be displaced from the actual relative heading by an amount equal to the error in the zero reading.
A second constant in relative heading sensors is the conversion factor between the output signal of the sensor and the relative heading. Like the zero reading, this conversion factor is subject to the influence of temperature and other environmental factors, and may have an even more profound effect upon the accuracy of the sensor. For example, a 10% error in the conversion factor would result in a 10% error in all relative heading measurements, regardless of magnitude. It has been found that errors in zero reading and conversion factor produce significant inaccuracies in heading measurement in automobile navigation systems.
It has been recognized, therefore, that the absolute and relative heading sensors used in automobile guidance systems must be calibrated regularly and accurately. On method for compensating for the magnetic compass or geomagnetic sensor output is shown in U.S. Pat. Nos. 4,611,293 and 4,797,841. Upon entering a vehicle, the user triggers an input to put the system into calibration mode. The car is then turned in a complete circle, with the system recording the X and Y points as the car is turned through a complete circle. The X and Y outputs are recorded for all angles of the turn, and the maximum and minimum values of the X and Y outputs are determined to allow determination of the axes of the ellipse. The system then determines the difference between the points on the ellipse and a perfect circle for each output and uses these as a correction factor for that output. U.S. Pat. No. 4,072,565 suggests at col. 4, lines 15-19, that a separate sensor could be used to detect a revolution of the vehicle for such a calibration method.
Another method for calculating a geomagnetic sensor is to use the output from an angular rate sensor to provide a correction. Such systems are shown in U.S. Pat. Nos. 4,831,563 and 4,862,398. However, as is apparent from the above discussion, the angular rate sensor itself needs to be calibrated.
A conventional method of calibrating a relative heading sensor, such as a gyroscope, is to sample the sensor output when the vehicle is standing still to obtain a zero reading. Then, in order to calibrate the conversion constant, the sensor is placed on a turntable turning at a constant speed. The sensor output is compared with the speed, and the conversion factor is determined by dividing the speed by the gyroscope voltage output. However, this method of calibrating a relative heading sensor is not practical for vehicle navigation, since it would require removing the sensor and rotating it on a turntable. This method further requires specialized calibration (e.g. a turntable) equipment which may not be readily available.
Another approach to calibration of a relative heading sensor utilizes a pre-calibrated absolute heading sensor, such as a magnetic compass, mounted in a fixed position relative to the relative heading sensor. As the relative heading sensor is rotated, the difference between two headings recorded by the absolute heading sensor is compared with the heading displacement measured by the relative heading sensor in order to calculate a conversion factor. This method is advantageous where the relative heading sensor is mounted in a vehicle and cannot be easily removed. However, the method requires the use of a second sensor such as a compass, and the second sensor must be calibrated before the relative heading sensor may be calibrated.
For these and other reasons, a method of calibrating a relative heading sensor in a vehicle navigation system is desired which does not require additional calibration equipment or removal of the sensor from the vehicle. It would be further desirable if the calibration method allowed calibration of the relative heading sensor without requiring the use of a pre-calibrated absolute heading sensor. In particular, the calibration method should permit calibration of both a relative heading sensor and an absolute heading sensor at the same time and during the same procedure. Finally, the calibration method should be simple to perform in view of the frequency that such calibration is required in automobile navigation systems.